Use True for the Exclusive parameter:
InputOptionPage := CreateInputOptionPage(wpWelcome, 'DIS_SERVICE SELECTION', '', 'Please select the service', True, False);
See:
Inno Setup Help[
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Quote:
Open the "CodeDlg.iss" script in the "Examples" subdirectory of your Inno Setup directory for an example of how to use pre-built custom wizard pages and event functions. Open the "CodeClasses.iss" script for an example of how to use completely custom wizard pages and controls.
Inno Setup Help[
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